A manhole exploded hundreds of feet into the air – what's the science behind this?

A manhole exploded on a road in Buffalo, New York, last weekend. Odd, sure, but that's the science behind this? 

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The unsuspecting people of Buffalo, New York, were enjoying a warm sunny Sunday afternoon, when all of a sudden, smoke spewed through the slits of manholes on an intersection. A witness, in a state of fear and bewilderment, reported the smoke at about 11:30am. The police closed off the road and had two buildings evacuated, suspecting an underground electrical fire. 

On his way to the scene, photojournalist Dave Harrington of WGRZ heard a loud noise, and started interviewing a witness, when...“BANG!” A second explosion sent a 50kg manhole, the weight of a Giant octopus, 200-300 feet into the air. Harrington’s camera caught the whole thing:

Later on, at about 2pm, there was a third, smaller explosion. Steve Brady, a spokesperson for the National Grid, told WGRZ that electrical wires had caught fire, but the cause was still under investigation. Nothing will be known until they pull up some burned cables, which could take a few days. 

Here’s another example, in Birmingham on 22 January 2015, at about 12.21pm, where a child was just inches from a manhole explosion:

A National Grid representative told the BBC that such explosions did happen, but were “not common”. 

But what’s the science behind exploding pavements and roads?

Underground cables fray with age  this could be from corrosive chemicals, overload, or pesky critters like street rats thinking they’re food. These cables carry an extremely high voltage  around the order of 13,000 volts of electricity. Damage to these cables cause wires to touch and heat up the insulating material, and possibly the outside debris too.

The insulating material starts to smoulder and catch fire, and as that spreads combustible gases like methane, hydrogen and carbon monoxide are released. As a result, pressure builds up inside the manhole or maintenance tunnel. The manhole is now a ticking time bomb, and like a bomb, the electrical wires act like a fuse that can ignite the gas with a bolt of electricity; this can cause a powerful explosion – sometimes with fire. 

If there’s good ventilation, then the fire might smoulder out into nothing – there will be burned and broken cables (or even maybe a larger fire) that need dealing with, but no explosion. The problem is when gas builds up, as the greater the pressure then the greater the potential energy of any explosion, and the greater the impact it has on the thing that usually acts as the first point to break: the manhole cover.

A manhole cover weighing between 35 to 136kg that experiences a sub-surface pavement explosion can be propelled between one to 50 feet into the air. The explosion in Buffalo is an exception – it propels the manhole cover at a distance far above the average.   

A number of utilities in the United States have started to install slotted manhole covers to allow gases to be released a lot less violently. This is reassuring – just imagine if one of these hit somebody on the way down.

Tosin Thompson writes about science and was the New Statesman's 2015 Wellcome Trust Scholar. 

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